2 INSECTICIDES, FUNGICIDES, AND WEED KILLERS. 



which greatly influence plant growth. They cause deadly plant 

 diseases when all favourable conditions do not work together. Here, 

 as in the animal kingdona, the great law of selection intervenes. 

 Only plants able to live in the existing climatic conditions will 

 subsist, the others suffer, fall ill, and disappear. There are few 

 remedies for climatic causes. 



(2) Chemical Causes. — These relate, especially, to the chemical 

 composition of the soil, from which the plant draws the elements 

 required for life and growth. These may exist too abundantly, or not 

 in sufficient amount. Ee terms those diseases due to an excessive 

 amount of nutritive elements sthenic diseases, and those due to an in- 

 sufficient amount asthenic diseases. These physical and chemical 

 causes favour the hatching of parasitic diseases. Plants weakened by 

 these causes can exert little or no cellular resistance to being overrun 

 by parasites, which find in them a medium favourable to their growth 

 and development, for however unfavourable these conditions may be 

 to the development of the plant, they are, precisely, those which 

 favour the evolution and multiplication of parasites. 



(3) Parasitic Causes. — The evolution of all parasites, animal or 

 vegetable, should be well known, as they are neither injurious nor 

 destructive to the same extent in different phases of that evolution. 

 Each parasite has a very characteristic evolution, more or less long, 

 always the same for the same species. (1) Animal Parasites.- — These 

 belong to the most diverse classes, but insects especially occasion the 

 most frequent and perceptible damage. The insect originates in the 

 egg. The insect which it contains passes through very different con- 

 ditions, until after a greater or less length of time it becomes perfect, 

 that is, capable of laying eggs to ensure its reproduction. The 

 sequence of these intermediate conditions is called metamorphosis, 

 and the different forms, larvae, caterpillai'S, grubs, chrysales, etc. It is 

 especially in the larva form that the insect is most injurious, because 

 it sometimes remains several years in that condition. That form 

 represents in every instance the longest period in the life of the insect. 

 A sound knowledge of these metamorphoses, of the periods at which 

 they take place, and the spots where they occur will render the 

 struggle against insects at the moment of their evolution — when they 

 are the most sensitive to insecticides and when they cannot withstand 

 their energetic action — comparatively easy. (2) Vegetable Parasites. 

 — These include fungi and certain phanerogams. Fungi have 

 also an evolution, differing greatly with the species, and often very 

 complex. Their method of reproduction, the time and place where the 

 spores are produced as well as the plants on which these spores can 

 germinate, should be known. The elements of reproduction of fungi 

 are spores (name given to seeds of fungi). These spores develop 

 when climatic conditions permitting their evolution are realized. 

 The mycelium (this is the name given to the new-born parasite) can then 

 expand, either in the interior or on the exterior of the plant. In both 

 cases it lives at the expense of the cellular tissue, and in so doing pro- 

 duces characteristic diseases. Organs of reproduction are formed on 

 the mycelium, from which spores that spread the disease are de- 



